Highwall mining is a procedure for extracting coal or other mineral from a seam with remote control machinery which extends into the seam from a bench at the surface of a highwall. A highwall is a vertical or steeply sloped face of an excavation which has been cut on a mountain side or has been exposed by the digging of a pit in the course of a surface mining operation. The portion of the seam which has been removed by the mining excavation will leave a relatively flat and generally horizontal surface at the base of the highwall referred to as the bench. As a result, at the base of the highwall will be found an artificial outcrop of coal or other mineral underlying an overburden of soil and rock. From the outcrop a continuation of the mined seam will extend into the earth or under the mountain.
The creation of highwalls is in part due to the economic factors involved in strip mining. The cost of exposing and removing coal from a seam is related to the amount of overburden which must be removed to expose the seam. As the surface mining operation progresses, if the overburden progressively thickens, it eventually ceases to be economical to continue surface mining. This is because the amount of overburden material which must be removed per given amount of coal recovered reaches a ratio at which it is no longer cost effective to proceed. As a result, the seams of the coal remaining beyond the face of the highwall must be mined, if at all, by other techniques.
With seams of this type, the thickness is frequently too small to allow the mining to be economically carried out by miners working within the seam. Often, the seam may be no more than two to three feet in thickness. For this reason, the art of highwall mining is undertaken by the use of remote control machinery.
Early efforts to mine a coal seam from a highwall included the use of coal augers to bore circular holes in the coal seam. Auger mining was employed to mine coal seams from benches cut in mountainsides or left from surface mining operations. These auger devices, however, made poor utilization of the natural coal. Often the coal removed was contaminated with material from outside the seam since there was no effective method for guiding the advancing auger.
Other developments in highwall mining involved the use of continuous mining machines which were remotely controlled to proceed into a coal seam. These machines were followed into the mined hole by open conveyor vehicles which removed the coal to the surface. Such devices, as for example one referred to as the Push Button Miner developed by Joy Manufacturing Co., were only marginally effective. They experienced a serious problem of the expense of recovering machinery after roof falls during mining and they were unable to accommodate the wide variety of roof conditions encountered. Accordingly, this prior method has been practically abandoned.
Furthermore, mining techniques which employ open conveyors to transport the mined coal from the seam to the surface also allow the rock which falls from the roof to fall onto the conveyor to be removed with the coal. This fallen roof rock, so removed, contaminates the coal, increasing its ash content and thus reducing its market value.
More recent developments in highwall mining machines have been more successful. One such machine is disclosed in U.S. Pat. No. Re 31,622 issued to Robert E. Todd. Machines such as those shown in Todd Reissue Pat. No. 31,622 represent a substantial improvement over prior equipment and methods for mining thin seams from a highwall.
The Todd miner includes a cutter head which is driven into the coal seam by a rectangular compression beam which is assembled in sections behind the mining head and is thrust from a baseframe at a bench outside of the mined hole at the surface of the highwall. The mining head is similar to that of a continuous mining machine. Such machines permit deep entry into the coal seam, efficient recovery of resources and economical production of coal.
However, even though the Todd highwall miner has been constructed with a relatively smooth profile to aid in its extraction from the mined hole, in areas where the overburden is unstable, the operation of such miners has been hampered by roof material falling upon the cutter head and upon the pushbeams. After falling upon the equipment, such material will continue to rest upon the pushbeams and accumulate as the cutter head advances to its maximum extent into the seam. Then, as the mining machine is retracted from the mined hole, the fallen material resting on top of the pushbeams has a tendency to snag against the roof as the beams are extracted from the hole.
The height of the pushbeams is a minimum height sufficient to resist bending and house an adequately sized conveyor within. The cutter head is often larger than the pushbeams, especially at its forward end. Thus the snagged roof material on top of the pushbeams has a tendency to jam against the cutter head as it is withdrawn. As a result, removal of the equipment can result in damage to the cutter head or to the pushbeams or to various components of the machine. With newer, wider and more efficient cutter heads, roof falling tends to be a greater problem. Higher cutting rates and increased power permit cutting outside the seam and often results in a less smooth profile. As a result, the problem of jamming due to fallen roof material has become an increasing problem.
Because it is quite common to find a layer of unstable slate or shale overlying a seam of coal, the problems of roof fall into the mined holes in coal seams is a common problem. Often, the rock overburden which falls does so in relatively large sections of, for example, one or two meters across. As such, the tendency of fallen rock to jam the equipment is serious and the ability of the equipment to crush the fallen roof material so as to free the equipment has been ineffective or extremely time consuming.
The pushbeams of the Todd miner include a rectangular housing with a conveyor located within. The conveyor is in the form of a pair of rotating augers which transport the mined coal to the surface. These augers are of the type having spirals corresponding in direction to the direction of rotation so as to move material toward the surface at the highwall.
Any jamming of the mining machine in a mined hole results in a delay in extracting the machine and hence in a loss of the use of the equipment and a loss of time of the personnel operating the equipment or otherwise assisting in the mining operation. As a result, any jamming which occurs increases the mine cost of the coal.
Accordingly, there has been and remains a need to remove fallen roof rock from the surface of the miner, and to do so in such a way as to prevent its jamming of the mining equipment in the hole and to avoid contamination of the coal with the fallen rock.